Nocardia water-soluble mitogen (NWSM) is a mitogen which activates human B lymphocytes even in the absence of T cells. Tonsil and peripheral blood lymphocytes cultured in the presence of NWSM gave rise to large numbers of cells containing cytoplasmic immunoglobulins of IgM, IgA or IgG classes. The number of plasma cells appeared to be elevated as early as day 3 of culture and peaked on day 5 or 7. The IgM response preceded the development of cells containing other Ig classes, and was found to remain predominant on days 5 and 7. A similar number of plasma cells were generated in T-deprived suspensions; however, a quantitative regulation by T cells cannot yet be excluded. The response to pokeweed mitogen appeared to be similar, except that it was often less intense and not observed in the absence of T cells. Very few or no IgD-containing cells were found to be differentiated with both mitogens.
Soluble receptors for FcIgG released from unstimulated human peripheral blood lymphocytes were isolated by affinity chromatography on Sepharose 4B-IgG. This material was shown to interfere with the differentiation of peripheral blood B cells into Ig-secreting cells in cultures stimulated with pokeweek or Nocardia opaca extracts. Neither cell viability nor [3H]thymidine incorporation were altered, but the number of Ig-containing cells and that of Ig-secreting cells were decreased. These effects were dose-related. They were found to be associated with Fc IgG-binding soluble material, since absorption on Sepharose 4B-IgG but not on pepsin-digested F(ab')2 fragments removed the inhibitory activity. This suppressor factor, released by unstimulated lymphocytes, may represent a human analogue of murine immunoglobulin-binding factor (IBF) produced by alloactivated T cells.
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